1. Field of the Invention
This invention relates to a process for continuously producing a high-melt viscoelastic ethylene-propylene copolymer. More particularly it relates to the process wherein a polymerization step (i) using propylene as a main component is carried out in the first polymerization vessel to the third polymerization vessel or more among 4 or more polymerization vessels connected in series, and a polymerization step (ii) using a relatively large quantity of ethylene is then carried out in the fourth polymerization vessel et seq.
The copolymer obtained according to the process of the present invention has a superior high-impact strength to that of similar products obtained according to conventional process, and is suitable for post-processed sheet and for blow molding.
2. Description of the Related Art
Sheets produced by process-molding known general-purpose polypropylene, when used for post-processing, have the following various drawbacks:
(1) The sheets rapidly sag at the time of molding for its conversion; the ranges of processing conditions are narrow; and the molding efficiency is inferior. Besides, (2) in the case of broad sheets, the above sagging is to a large extent; the thickness of post-processed products is liable to be ununiform; and accumulated wrinkles occur. Thus it is possible to produce only small molded products.
On the other hand, in the case where known general-purpose polypropylene is processed according to blow molding process, the following problems have been raised:
(1) Since sagging of parison at the time of molding is so large that the thickness of the resulting molded products is ununiform. Thus, blow molding process can be applied only to small molded products.
(2) If a high molecular weight polypropylene is used for preventing the above sagging, dangers of causing inferior melt fluidity, increase in the load and increase in the energy loss at the time of molding, and other mechanical troubles not only increase, but also the surface-roughening of practically molded products is so notable that the commodity value thereof is lost.
In order to improve the above sheet-moldability and blow moldability at the time of use of polypropylene, for example Japanese patent publication No. Sho 47-80614/1972 and Japanese patent application laid-open No. Sho 50-8848/1975 disclose blending of a low density polyethylene with polypropylene. However, molded products obtained by using such a blend are liable to cause surface-roughening, and in order to prevent the surface-roughening, powerful kneading is required at the time of melting so that the practice of these inventions is restricted in the aspect of choice of kneader and power consumption. Besides, the resulting invented products also have a problem of reduction in stiffness. Further, Japanese patent application laid-open Nos. Sho 53-91954/1978, Sho 57-185336/1982, Sho 57-187337/1972 and Sho 58-7439/1983 propose a process of kneading polypropylenes having different molecular weights by means of a granule-making machine or the like. However, the resulting molded products obtained from the blends of these inventions are liable to cause more surface-roughening than that in the case of the molded products from the above-mentioned blends of low density polyethylene; thus the practice of these inventions is restricted in the aspect of kneading process and choice of molecular weight difference.
In order to solve the above-mentioned problems relative to the moldability of general-purpose polypropylene, a number of processes of broadening the molecular weight distribution of the objective products according to a multi-stage polymerization process have been proposed (e.g. Japanese patent application laid-open Nos. Sho 57-185304/1982, Sho 57-190006/1982, Sho 58-7405/1983, Sho 58-7409/1983, Sho 59-172507/1984, etc.). However, according to most of Examples of these inventions, propylenes having different molecular weights are produced at a multi-stage according to batch polymerization process, but the so-called idletime takes place which has essentially nothing to do with the polymerization reaction itself, such as times required for feeding raw material, withdrawing product, etc.; thus as a commercial production process, the above process has a drawback that it has a low productivity per unit equipment.
Further, the above inventions of Japanese patent application laid-open No. Sho 57-185304/1982 et seq, refer also to a continuous polymerization process, and when the order of the stepwise production is directed to (a) a combination of a high molecular weight product.fwdarw.a low molecular weight product, it is possible to achieve the necessary molecular weight difference only by adding hydrogen at the later stage of the multi-stage polymerization; hence the case (a) is preferred as process, whereas to the contrary when the order is directed to (b) a combination of a low molecular weight product.fwdarw. a high molecular weight product, it is required to remove unnecessary hydrogen by pressure drop or degassing of the inside of the vessel holding the polymerization reaction mixture after preparation of the low molecular weight product and before preparation of the high molecular weight product; thus it is described therein that the processability in the case (b) is inferior to that in the case (a).
However, according to a tracing experiment of the present inventors directed to the above process, in the case of production in the order of a high molecular weight product.fwdarw.a low molecular weight product (the above references describing that the former order is preferred), if the melt flow rate (hereinafter referred to as MFR) value of the high molecular weight part is low, a problem is raised that its measurement is difficult or impossible so that there is an obstacle in the aspect of operation control (Note: measurement of viscosity [.eta.] is possible, but since a long time is required for the measurement, such a process is not practical as an operation control means).
Further, it has been found that polypropylene produced at a multi-stage in the order of a high molecular weight.fwdarw.a low molecular weight has an abnormally large difference between the MFR value of powder prior to granulation and the MFR value of pellet obtained by granulation (note: the MFR value of powder is lower); thus there are problems in the aspect of control of the differences between the molecular weights at the respective stages and control of the MFR value of the resulting product.
On the other hand, crystalline polypropylene has superior physical properties in the aspect of stiffness, heat resistance, etc., whereas it has a problem in that its high-impact strength and particularly its low temperature high-impact strength are low; thus in this respect, the range of its practical uses has been restricted. In order to overcome this drawback, a number of processes of block-copolymerizing ethylene or another .alpha.-olefin with propylene have been proposed (e.g. Japanese patent application laid-open Nos. Sho 50-142652/1975, Sho 52-8094/1977, Sho 57-34112/1982, etc.). However, according to such a process, polymerization (1) of monomers composed mainly of propylene is carried out, followed by removing hydrogen and then carrying out polymerization (2) of monomers containing a relatively large quantity of ethylene, at that time a device for broadening the molecular weight distribution being carried out.
In general, when a multi-stage continuous polymerization process is employed in a block copolymerization process, there occur distributions in the retention times of the respective catalyst particles at the respective stages (which distribution is assumed to be close to a complete mixing vessel distribution); thus the resulting product corresponds to an aggregate of polymer particles wherein the proportions of polypropylene portion and polyethylene portion (which contains a relatively large quantity of ethylene) contained therein vary each time, and hence there occurs a drawback in the aspect of product quality due to the above-mentioned ununiformity.
Particularly in the process of the present invention described later in detail, since molecular weight differences are imparted at the respective stages in the multi-stage polymerization step (i) wherein monomers composed mainly of propylene are polymerized, if a known art is employed as it is, molecular weight differences between the respective polymer particles are enlarged as compared with the case of conventional block copolymerization so that the problem due to the uniformity becomes more serious.
A number of processes for overcoming the above drawbacks of the multi-stage continuous polymerization process according to the prior art have been proposed. For example according to Japanese patent application laid-open Nos. Sho 58-48916/1983 , Sho 55-116716/1980, Sho 58-69215/1983, etc., a slurry having left a propylene polymerization part (polymerization step 1) is subjected to classification by means of a cyclone and the resulting fine particles are again returned to the propylene polymerization part. However, since the classification of polymer particle sizes does not always accord with the retention time distribution, the effectiveness of the process is insufficient. Further, according to Japanese patent application laid-open Nos. Sho 57-195718/1982 and Sho 58-29811/1983, catalyst feed to a polymerization vessel and slurry withdrawal from the vessel are intermittently carried out, whereby the catalyst portion entering an ethylene polymerization part (polymerization step 2) is reduced during a short period of the retention time.
However, such a process has a drawback that the polymerization reaction is unstable.
Further, certain processes similar to an embodiment (6) of the present invention described later have been proposed wherein a slurry having left a propylene polymerization part is treated with an electron-donative compound, etc. whereby the slurry is withdrawn in a short retention time and the catalyst particles are selectively inactivated.
For example, Japanese patent application laid-open Nos. Sho 57-145115/1982 and Sho 55-115417/1980 propose various electron-donative compounds, but compounds in the range of those used in Examples are insufficient for achieving the object of the present invention described later.
In view of the above-described drawbacks of the prior art directed to a multi-stage continuous polymerization process for producing a propylene-ethylene block copolymer, the present inventors have made extensive research in order to find a polymerization process having overcome the drawbacks, and as a result have found that when (1) polymerization using propylene as a main component is carried out in the initial three polymerization vessels or more among four polymerization vessels or more connected in series (ethylene content in monomers fed: 0 to 5% by weight), and successively polymerization using a relatively large quantity of ethylene is carried out in one or more remaining vessels (ethylene content in monomers fed: 10 to 100% by weight), and (2) the total quantities of a catalyst used and hydrogen as a molecular weight modifier are fed to the first polymerization vessel and the catalyst and hydrogen (excluding their quantities consumed midway) are successively transferred to the second polymerization vessel et seq together with the reaction mixture (slurry), then it is possible to produce an ethylene-propylene block copolymer having a broad polymerization degree distribution stably and with a good reproducibility; thus it is possible to substantially overcome the above-mentioned drawbacks of the prior art.
As apparent from the foregoing, an object of the present invention is to provide a continuous polymerization process for producing a high-melt viscoelastic ethylene-propylene copolymer, which process makes it possible to stably produce an ethylene-propylene block copolymer having superior physical properties suitable to sheet molding and blow molding and capable of producing large-size molded products. Another object of the present invention is to provide a copolymer produced according to the above-mentioned process. Other objects of the present invention will be apparent from the succeeding description.